Another option: pre-fill some content with some FB or Twitter content. Then you have "users". Whenever a user's "front' page is visited, increase that user's

Another option: pre-fill some content with some FB or Twitter content. Then you have "users". Whenever a user's "front' page is visited, increase that user's counter, and then display popular users on the main page. Sure, you don't have true "true active" users, but at least it's users that generate some activity, indirectly. If some users become popular, chances are they'll want to have a look at your service. This might be a very bad idea, your call.